A satellite power-system generally consists of a solar array (which is a set of solar cells connected in series and parallel) to generate electrical energy from sunlight and provide the source to power the spacecraft. The on-board users of this power are the housekeeping subsystems (required to operate the spacecraft, such as the attitude control and the telecommand) and the actual payload (the purpose of the mission e.g. telephone, television, scientific, etc). In addition, a certain power is required to charge the storage battery so that if, at any time, the user power exceeds the solar array power the excess can be provided by the battery. This can happen for example during eclipse when there is no solar array power or during a period of peak power demand. The power capacity of a solar array depends linearly on its area and can be expressed in watts per square metre. Over its lifetime, this power-capacity degrades with radiation effects, so that its size is generally based upon its power capacity at end-of-life. Because the solar array represents a very high proportion of the financial cost and mass of a satellite, any technique to reduce these is most important.
The current-voltage (I-V) characteristic of a solar array is shown in FIG. 1. The short circuit point 1 represents the short circuit situation in which the short circuit current I.sub.sc is output. The open circuit point 3 represents the situation when the open circuit voltage V.sub.oc is output. At some intermediate position 5, the solar array operates to produce its maximum power, at the maximum power point (MPP).
One system for regulating the voltage of a solar array is known from U.S. Pat. No. 4,186,336 (Weinberg et al). This patent describes a low dissipation series regulator that maintains a constant voltage at a load by sequentially switching elements of a solar array, as will now be explained.
In this prior art, the solar array is divided into a plurality of sections, each of which can be sequentially switched by a single control voltage. Each solar array switch is driven by a comparator with two predetermined hysteresis thresholds. The two thresholds are at very similar voltage levels, but are different for every comparator, so that as the control voltage varies the switches open or close at different voltages so that the number of solar array elements connected to the output is-varied as a function of control voltage. This control voltage is generated by the output from a differential amplifier which amplifies the difference between a signal proportional to the solar array output voltage and a reference voltage. By this means the output voltage is maintained at a predetermined value. A smoothing capacitor is connected across the output. In use, a load is connected to the output, and draws a current tending to discharge the capacitor. The solar array elements act as current sources tending to charge the capacitor, and the correct number of elements are sequentially switched into operation to maintain the capacitor at a predetermined constant voltage. The other solar array elements do not supply power.
However, in this technique the solar array will often be operated far from its maximum power point, since the output voltage is constant and yet the voltage at the maximum power point is highly variable. Thus, the maximum potential power of the solar array cannot be utilised.
Methods of operation of a solar array at its maximum power point (MPP) are described in "A maximum power point tracker for a regulated power bus", Teulings et al., Proceedings of the European Space Power conference pages 93-97, Graz, Austria, August 1993, and in "Advanced power conditioning using a maximum power point tracking system", A.Poncin, Spacecraft electric power conditioning seminar, Frascati, Italy, May 1974, pages 75-86.
The solar array must be designed to supply power equal to the average power demand including peaks of the demand taken over the total eclipse-sunlight cycle.
In most prior art techniques the solar array is controlled to be at its MPP by variation of the power required to charge or discharge the on-board storage battery of the satellite. By this means the peak power demand can be supplied from the battery and the battery recharged when this peak demand disappears. To illustrate the principle a very basic scheme for achieving this is shown in FIG. 2. The DC/DC converter or converters 9 produce the current to charge the battery 11 and supply the user load 13; this power comes from the solar array 7 via the bus 19. Normally there is more than one such converter, although for simplicity only one is shown in FIG. 2. By variation of the battery charge current and discharge current the solar array can be regulated to operate at its MPP.
However, the MPP technique of the prior art has the following problems:
a) An on-board battery is required, of sufficient capacity to absorb the excess power of the solar array and supply the peak load demand. This can result in a battery size and cost much larger than that required for eclipse operation.
b) It may not be possible to vary the current into the battery according to the requirements of MPP tracking. For example, when the battery is being reconditioned, or is fully charged, it is effectively disconnected from the solar array. Under these conditions the MPP tracker cannot store the excess power of the solar array, which means the solar array voltage is not controlled to be at the MPP and will be forced to operate near to the open circuit voltage of the solar array. This solar array open circuit voltage can be very high especially for an array at low temperature when just emerging from eclipse and at beginning of life (between 2 to 3 times nominal). The result is that any pieces of equipment connected to this solar array voltage (i.e. battery charge DC/DC converters) have to be rated for a large voltage variation at their input. The impact of this is a lowering of the efficiency which together with the higher voltage rating required by the components of this equipment results in them having a higher mass and cost.
c) The DC/DC converter(s) used to charge the battery and supply the load have to be rated for the total spacecraft and battery recharge power. This results in higher mass and cost than a converter that only handles the battery recharge power.
d) The transient output impedance of the MPP Bus 19 is relatively high for fast load changes because the technique of regulating this Bus has to be slow. This is due to the low frequency filters required for the DC/DC converter used to implement MPP tracking. This means it is difficult to connect the user directly to the VMPP Bus without using special methods (such as additional DC/DC converters). This in turn means that normally all the user loads are connected across the storage battery resulting in problems (e) and (f).
e) The battery has a relatively large output impedance (although not as large as the solar array MPP bus) and since the battery is the common point of all the loads, noise problems can result due to high ripple voltage.
f) The peak power demand must come from the battery resulting in a lower battery life due to more charge/discharge cycles, and the requirement for the battery to be able to supply higher peak current.
g) The control techniques required to perform the function of keeping the solar array at its MPP and the battery properly charged is combined in the DC/DC converter which results in a control of considerable complexity.
The example of prior art given is the very simple case of supplying the users from a battery,; there are other techniques where the user is also supplied from the MPP bus through converters as shown dotted in FIG. 2 (an extra converter 17 to supply the second load 15). These extra converters are used to overcome some of the problems discussed above for the prior art but the penalty paid for these additions is mass, cost and complexity. In all these configurations, the same principle is used for MPP tracking. This principle is to keep (whenever possible) the solar array operating at its MPP, by variation of the charge and discharge current of the battery and variation of the power demanded by the user's loads.
There is thus a need for a solar array system that reduces the disadvantages of the prior art systems discussed above.